1. Field
This application generally relates to the field of wireless communication systems, and more particularly to enhancing data transmission efficiency in such communication systems.
2. Related Art
The subject matter set forth herein is applicable to wireless communication systems generally. However, it has been developed primarily in the context of cellular telecommunication systems, which facilitate high-speed connectivity and data and voice transport on both point-to-point and point-to-multipoint bases. First-generation (analog) and second-generation (digital) cellular networks were used primarily for communicating voice traffic via mobile cellular telephones, and thus maintained a focus on access methods for the efficient transport of voice information. With the rising popularity of the Internet, a third-generation (3G) wideband multimedia cellular network has been proposed that transports both voice and data at much higher speeds than were previously available using the first and second generation wireless networks.
A Third Generation Partnership Project 2 (3GPP2) has been established by industry groups for the purpose of defining specifications to transition current code-division multiple-access (CDMA) wireless networks to the third generation, which is commonly referred to as CDMA2000. One such specification may be referred to as “CDMA2000 1×Revision D” (which may also be referred to as “CDMA2000 1×Rev D,” “cdma2000 Release D,” “IS-2000-D”, or “IS-2000-Rel. D”). The CDMA2000 1× Rev D specification, available from the 3GPP2, is incorporated by reference herein in its entirety for its teachings on communications protocols used in 3G wireless communications systems.
Recently, many proposals of communication protocols for use in CDMA2000 Release D have been submitted to the 3GPP2, including protocols for both forward link transmissions from a base station (BS) to a mobile station (MS), and for reverse link transmissions from an MS to a BS. Most of the proposals pertaining to reverse communications links for use in cdma2000 Release D communication systems recommend a pilot channel transmission level “boosting” scheme using a Reverse Secondary Pilot Channel (R-SPICH).
The R-SPICH is a pilot channel that is additional to a primary reverse pilot channel (R-PICH), and which may be used in CDMA2000 Release D systems to aid in decoding of the new high-speed reverse packet data channel (R-PDCH). It is generally preferred to transmit the R-PICH at the same levels that were employed according to previous releases, thus maintaining backward compatibility. The R-SPICH is not burdened with backward compatibility limitations. As such, transmission power levels of the R-SPICH may be made variable, based for example on the data rates of the reverse packet data channels (R-PDCH) with which they are associated. Thus, the R-SPICH may be combined with the R-PICH to enhance, in steps, the precision of a coherent demodulation phase reference that is formed by combining the R-PICH and the R-SPICH, yet without jeopardizing compatibility with system components that have been configured in accordance with earlier releases of the standard.
Because transmission power on one CDMA channel appears as noise on other channels, it is generally desirable to transmit at the lowest power that will achieve adequate reception accuracy. The optimal transmission level of an R-SPICH depends upon the data rates and channel conditions of the data channel(s) with which it is associated. The associated data channels may achieve higher data rates by employing more efficient coding schemes and modulation techniques. Corresponding improvements in the accuracy of the channel estimates, the phase reference, and/or the data signal estimate, can facilitate demodulation and decoding at such increased code densities.
A transmitting MS may determine a rate at which to transmit data, and may select transmission characteristics, such as coding and/or modulation techniques, to achieve the selected data rate. To support accurate reception, the MS may also select an appropriate transmission level for the R-SPICH. Selection of these factors by the transmitting MS requires less signaling overhead than, for example, requiring a sequence of communications by which the receiving BS grants permission to the MS to employ such factors. However, in order to achieve optimal combining of the R-SPICH and the R-PICH, the receiving BS should acquire information regarding the transmission level of the R-SPICH, as compared to the R-PICH level.
Various recommendations have been proposed to the 3GPP2 for conveying R-SPICH transmission level information to the BS, but each has one or more drawbacks. One proposal recommends transmitting information to indicated relative transmission levels of the SPICH in a companion signal channel. Disadvantageously, this requires an additional signal channel, or imposes a data burden on an existing signal channel; moreover, combining can then only be performed after the control channel is decoded, which can delay the combining process for at least four power control group (PCG) time periods, or a time period of 4 (1.25 ms). Another proposal suggests that information be transmitted ahead of time or based on a previously transmitted frame. Disadvantageously, error propagation of incorrectly detected rate information can occur using this approach, and the information may also be stale, or at least not optimally up-to-date. Another proposal recommends that the SPICH channel not be multi-level, but rather be either on or off, with nearly “real-time” SPICH detection. Disadvantageously, a single SPICH transmission level fails to efficiently match the anticipated wide range of data rates and channel conditions. Such coarse SPICH levels will either be suboptimal for the highest data rates, or will waste system resources when employed with intermediate data rates.
In order to efficiently boost overall pilot effectiveness by adding a secondary pilot channel, the first and second pilot channels should be combined. According to mathematics well known to those of skill in the art, the signals may be more optimally combined if a ratio of the power at which the two pilots are transmitted is known at the receiver. Moreover, in order to achieve optimal decoding, the Traffic to Pilot ratios (T/P, also referred to as “TPR”) should also be known. Each of the proposed methods described above exhibit some limitations in achieving optimal T/P ratios, in reducing the complexity of the receiver (due to information latency for BS optimal combining), or in achieving robust error tolerance.
Therefore, a need exists for a method and apparatus for boosting pilot channel transmission levels in a wireless communication system that overcomes the disadvantages of the previously proposed approaches. The disclosed enhancements address this need with techniques that are broadly applicable in many types of wireless systems to provide modest amounts of timely data, and which may be particularly useful to facilitate data reception in a system having variable transmission characteristics.